1. Field of the Invention
The invention pertains to measurement of optical quality of the surface of a transparent object. It is especially adapted to measuring the surface of flat glass, especially flat glass which is to be combined with a homologous glass to produce a laminated assembly.
2. Description of the Related Art
Whenever one desires to utilize the optical qualities of a transparent object, it becomes necessary to evaluate the surface characteristics of the object. Thus, if one desires to produce a plain mirror with high precision, to manufacture a heliostat from float glass, it becomes desirable to evaluate the optical quality of the back surface of the glass, which is to be silver plated.
Also, in the fabrication of an automobile laminated windshield, which in a modern car is highly sloped, one will try to combine pairs of sheets whose shapes are such that the transparent plastic material between them has a thickness as constant as possible, so as to avoid a lens effect. In this case it is important to know the irregularities of each surface so that the combination will be beneficial and in order to avoid degradation of the optical quality for each of the sheets of glass.
Also, in the technical field of flat screens, particularly those which utilize liquid crystals, one desires to combine two sheets of thin glass whose facing surface geometries are compatible. Indeed, it is important that the variations of thickness of the sandwiched sheet made of plasma or nematic material be limited.
Methods and devices for measuring by reflection the optical quality of transparent objects, sheets in particular, are well known. U.S. Pat. No. 4,585,343, for example, illustrates a system of continuous evaluation of the inherent flatness of sheets of glass at their output from a sheet hardening furnace. A beam of approximately parallel light is directed perpendicular to the sheet and one controls its variation of convergence after reflection. Such a system provides only gross information, and it is impossible to know what portion of the measured variation of convergence is due to each of the sides or possibly to defects in the glass itself. Moreover, the precision of the method and its power of spatial resolution are inadequate to measure a float glass which has not been subjected to a tempering process.
Optical measurement methods have been developed especially to obtain information on the quality of each of the surfaces of a transparent sheet, independently of one another. French patent FR-B-2,153,817 proposes, in addition to other innovations, a method for measuring the optical quality of a sample by observing the displacement of a luminous beam, the source and the detector being located on the same side. In order to separate the beams reflected by the two sides and to eliminate the one reflected by the back side, an oblique incidence is used and a screen is placed on the path of the second beam. This technique therefore obliges one to use oblique incidence. Another possibility consists in "immersing the second side in a liquid with the same index as the glass."
Another document, EP-A-0,485,043, uses two incident parallel beams and scans the four reflected beams, analysis of the signal allowing one to select the two pencil beams reflected by the front side.
The two preceding methods have the same disadvantages: the measurement is only carried out in one direction and a very precise relative motion (constant speed) is required between the sample to be measured and the apparatus.
The principle of the separation of beams of the preceding methods, due to the necessity of using narrow luminous pencil beams, cannot be applied in ombroscopy, a very interesting gross method because it allows one to detect the most significant defect, whatever its position and direction. Another advantage of ombroscopy is that it does not require relative movement between the sample and the measurement devices.
Another document, U.S. Pat. No. 3,857,637, also proposes to let the sample be moved in its plane and to follow the reflected beam. In order to avoid back reflection, the proposed technique consists in covering the back face of the sample with a coating which absorbs light, such as a dark paint. This is a laboratory technique, however, which is not adapted to fast and non-destructive production inspections.
Several methods for determining if two sheets of glass to be combined with one another with help of a transparent resin have surfaces of complementary shape or not are also known in the prior art.
U.S. Pat. No. 4,837,449 examines parallelism of facing internal surfaces in an assembly of two sheets. The position of a laser beam traversing the assembly is compared to that of the beam which has been successively reflected by the two surfaces that are to be in contact with the inserted resin before traversing the second sheet.
Another document, WO 90/02310, proposes a gap detector which allows one to scan with a laser beam an assembly of two sheets and to study the spots of the reflected beams which overlap when the glasses are parallel and move apart if there is a space anywhere between the two glasses. This method requires a large number of cameras to "auscultate" an entire sheet of glass, otherwise the measurement can only be carried out in one direction, the one perpendicular to the direction of the sweep.
The two preceding methods allow one to give an opinion on the assembly of two transparent sheets which are already combined with one another, but one would like to have a method which allows one to classify sheets in order to know in advance which ones will be capable of being paired while maintaining an acceptable level of quality.